1. Field of the invention
The present invention relates to an image treatment method and apparatus for treating images by digital signals. More particularly, the present invention relates to an image treatment method and apparatus for expressing half-tone images in a pseudo-manner by performing a quantization treatment of input data.
2. Related Background Art
Hitherto, apparatuses of the type described above such as laser beam printers (LBP) and ink jet type of printers employing a binary recording method in which recording dots are treated to be "whether printed or not" have been known. When a copying treatment of images of half-tone density such as photographs or half-tone dot original documents is performed with a copying machine which employs the above-described binary recording method, a method is employed in which a treatment for expressing a half-tone in a pseudo-manner is used, the read-out half-tone image data being treated using an image treatment circuit thereof is performed.
As an example of the method of treatment of the above-described type of pseudo-half-tone treatment, there is, at present, a so-called "dither method" which is the method widely used.
This dither method has an advantage that the above described type of pseudo-half-tone processing can be performed with a low cost since the structure of the hardware thereof is simple. However, this method raises the following problems:
1 In a case where the original document is a dot image such as a print, the quality of the image deteriorates due to generation of periodical fringes (moire) in the copied image. PA0 2 In a case where the original document contains line drawings and/or characters, sufficient reproductivity of the lines cannot be obtained, and thereby the quality of the image deteriorates.
There is a method of overcoming problem 1 by performing a smoothing treatment (spatial filtering treatment) upon the read-out half-tone image data. Furthermore, there is a method of overcoming problem 2 by performing an edge exaggerating treatment. However, with the above-described methods, it is difficult to obtain images exhibiting a sufficient productivity upon all of various images such as photographs, dot images, line drawings, and characters. Furthermore, the size of the circuit for performing the above-described treatment is larger. Therefore, the original advantages of the dither method can deteriorate.
To improve on this, as an example of a pseudo-half-tone treatment, there is a so-called "error diffusion method" which has recently attracted public attention.
This error diffusion method is a method in which the error in the density, which is generated when the input image data is binarized, between that of the input (not yet binarized) image data and that of the output (binarized) image data is diffused to the peripheral picture elements whereby the density can be secured. This method was published in "An Adaptive Algorithm for Spatial Grey Scale "SID. 75 Digest" literature by R. W. Floyd and L. Steinberg.
This error diffusion method exhibits a rather improved gradating performance and resolution with respect to the above-described dither method. On the other hand, this method raises problems that a specific fringe pattern can be generated in a portion where the density of the image is uniform, and/or granular noise can appear due to generation of dots in a diffused manner in highlight portions of the image.
In order to overcome these problems, a variety of methods have been disclosed in U.S. Pat. Nos. 4,876,610, 4,878,125 and 5,008,95, and U.S. patent application Ser. No. 192,601.
Furthermore, a method is disclosed in U.S. Pat. No. 4,958,236, wherein generation of lines at the boundary portions of the images is prevented when the image is divided into a plurality of regions and the thus-divided regions are each quantization-treated in the error diffusion method.
On the other hand, when an original image is read and it is binarized by the error diffusion method so as to be output by a printer, there is a problem that there is a blank area in which no dot is printed as shown in FIG. 15 if the density of the original image is in a low level. Furthermore, in the region next to such blank region, dots are, as shown in FIG. 15, printed successively. As described above, the reproduced image output after treatment using the error diffusion method raises the problem that excessive deterioration in the quality of image is generated in the highlight portion in which the density of the image is in a low level.